Remove echoprint-codegen. (it's no longer HEAD-only)

Q.v. https://github.com/mxcl/homebrew/commit/bccaa4a4b54b49e2c52128ae6fccf89bbb6d2368

I expect deleting echoprint-codegen will fix the warning I recently received when I tapped `homebrew/homebrew-headonly`:

```sh
brew tap homebrew/homebrew-headonly
# Cloning into '/usr/local/Library/Taps/homebrew-headonly'...
# remote: Counting objects: 70, done.
# remote: Compressing objects: 100% (58/58), done.
# remote: Total 70 (delta 18), reused 60 (delta 12)
# Unpacking objects: 100% (70/70), done.
# Checking connectivity... done
# Warning: Could not tap homebrew/headonly/echoprint-codegen over mxcl/master/echoprint-codegen
# Tapped 30 formula
```